Add PhoneWindowManager checks to NavBar KillTask runnable

This addresses the issue where binding NavBar kill app can cause a system crash by killing inappropriate system processes.

Change-Id: I48f4c9b48131ffc9d7efe6f4f58bcaecc0fea304

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/ExtensibleKeyButtonView.java